Background: Sedentary behaviour has emerged as a distinctive health behaviour paradigm, showing associations with markers of cardio-metabolic health independent of other lifestyle behaviours, such as physical activity. However, most investigations have been conducted in the general population without reference to specific risk factors and experimental research in humans is lacking. Aims: 1. Undertake narrative reviews investigating the effect of sedentary behaviour upon markers of cardio-metabolic health 2. Conduct a series of observational studies investigating the independent effects of sedentary behaviour on traditional and non-traditional markers of cardio- metabolic health. 3. Design and conduct an acute experimental study to establish whether reducing sitting time through regular bouts of non-sedentary activity improves cardio- metabolic health in women at high risk of T2DM. Key Findings: 1. Experimental studies demonstrate that regularly breaking sedentary behaviour with short bouts of walking imparts acute metabolic advantages. 2. Sedentary time was shown to have stronger associations with 2-hour glucose, triglycerides, HDL-cholesterol compared to moderate-to-vigorous physical activity (MVPA). 3. Sedentary time was associated with interleukin 6 (IL-6), heart, liver and visceral fat, independent of measured confounders, including glycaemia, adiposity and MVPA. 4. Compared to a prolonged bout of sitting, both standing and walking significantly reduced the glucose and insulin incremental area under the curve (iAUC) in women at high risk of T2DM. Conclusions: This thesis synthesizes the current evidence examining the relationship between sedentary behaviour and various health outcomes (Chapters Two and Three). Chapters Four, Five and Six investigate the independent effects of sedentary behaviour on markers of cardio-metabolic health through a series of observational studies. Chapter Seven examines the effectiveness of an experimental study and Chapter Eight discusses the implications of these findings whilst identifying areas for future research.